1. Field of the Invention
The present invention relates to semiconductor laser devices and, more particularly, is directed to a so-called surface-emitting semiconductor laser device with emission directed perpendicular to the surface of a semiconductor substrate, for example.
2. Description of the Prior Art
A semiconductor laser device is realized widely as a light source for an optical disc, an optical fiber communication and so on. Further, the semiconductor laser device is made coherent and high in output and also monolithically integrated with a function device such as an optical modulator or the like. Recently, it is desired in view of application to parallel optical information processing or large capacity parallel light transmission or the like in an optical computer and so on that the semiconductor laser device is made as a two-dimensional integration of large scale. However, the merit test of a conventional semiconductor laser device can not be carried out if its elements are separated. Therefore, it is very difficult for the conventional semiconductor laser to be monolithically integrated.
For example, when a light emission end surface is formed by vertical etching process or the like, since a laser light is emitted in parallel to the surface of a substrate of the semiconductor laser device, it can not be made as a two-dimensional integration as it is. Further, there occurs a so-called diverge or the like in which the emitted light is shielded by or reflected on the surface of the substrate to cause interference or the like.
On the contrary, a surface-emitting type semiconductor laser device in which a laser light is emitted in the direction perpendicular to the surface of its substrate has been noted as a semiconductor laser device which can be two-dimensionally integrated. As such a surface-emitting type laser device, there is employed such a structure that, for example, a reflection mirror surface is provided with angles of 45.degree. relative to the light-emitting surface of, for example, an ordinary semiconductor laser device and a laser light reflected on the reflection mirror surface is delivered in the direction perpendicular to the surface of its substrate. In order to make the surface-emitting type laser device as a monolithic structure, in general, by using an anisotropic dry etching process, for example, a semiconductor layer formed on a substrate is subjected to two anisotropic etching processes in the direction perpendicular to the substrate and in the inclined direction of about 45.degree. relative to the substrate to thereby form a laser emitting surface and a reflection mirror surface and hence a surface-emitting laser device can be obtained.
There are such reports when, for example, semiconductor laser devices using InP system and GaAs system are formed, used are a mass transport (Z. Liau et. al Appl. Phys. Lett. 46 (1985) p. 115) method and an ion beam assisted etching (IBAE) method (T. H. Windhorn et. al Appl. Phys. Lett. 48 (1986) p. 1675), a chemical etching method (A. J. Spring Thorpe Appl. Phys. Lett. 31 (1977) p. 524) or a reaction ion beam etching (RIBE) method (T. Yuasa et. al CLEO '88wo6 4/27) and so on. According to these laser devices, as, for example, described in the report using the above IBAE method, they are high in threshold value and poor in output as compare with a laser device using a general cleavage plane.
That is, in these methods it is difficult that particularly a reflection mirror surface by an anisotropic etching process in an inclined direction is formed with a flatness in the order of atomic layer and the inclined angle is selected with high accuracy. Further, there are caused such problems that the emitting angle of light is deviated from the vertical direction and an aberration appears.
On the other hand, there is proposed a surface-emitting type laser device with a vertical resonator structure in which, for example, a reflection surface, a cladding layer, an active layer, cladding layer and a reflective surface are sequentially laminated on a substrate to form a resonator in the direction perpendicular to the substrate surface to thereby emit a laser light in the vertical direction. In the semiconductor laser of lamination structure, since the resonator is formed in the vertical direction, it is impossible that the cavity (gain area length) is selected large enough and hence at present it is impossible to obtain a sufficiently high output light. Thus, such the surface-emitting laser device is not practically realized.
Further, a multi-beam semiconductor laser device is used as a light source of a high speed and high definition laser printer or utilized as a 2-beam laser or 4-beam laser, which is shown in FIG. 1 or 2 in a schematic large scale perspective fashion, for parallel-writing on and parallel-reading from an optical disc, a magneto-optical disc or the like. In this case, as an optical system which processes a plurality of beams, generally used is a single optical system of simple structure similar to the optical system used in an ordinary single beam laser device, so that when the distance between the emitted beams is wide, there are caused a lens aberration and so on. Therefore, it is necessary that the distance between the laser beams is selected to be a narrow distance so as to cause no problem in that optical system. Also, in view of the matching property thereof with the pitch of tracks on a disc surface, the narrow distance between the beams is desired.
In the case of a semiconductor laser device, especially a high output semiconductor laser device used for a magneto-optical disc, when a beam distance becomes narrower, thermal interference between adjacent beams proposes a problem (for example, "Electronic Communicate on Society Technical Report (DQE88-6" by, for example, Tsunekane et. al). In order to avoid this thermal interference, it is necessary to widen the beam distance. At present, the beam distance is widened to its limit at which the optical system used can operate normally. In FIG. 1 or 2, a laser beam distance LO is selected to be about 50.about.100 .mu.m.